1. Field of the Invention
This invention relates to a video camera having, for example, a low-speed shutter mode, and more particularly to a video camera that can always provide optimum exposure control of video signals.
2. Description of the Prior Art
Video cameras equipped with an electronic shutter having high-speed and low-speed shutter modes have been available for practical use. FIG. 1 is a circuit block diagram of one of these video cameras. In FIG. 1, an image injected through a lens 41 is photoelectrically converted by a CCD pickup element 42 and then sent to an A/D converter 45 through a sample-holding circuit 43 and an AGC (Automatic Gain Control) circuit 44. A digital signal output from the A/D converter 45 is supplied to a signal processor circuit 46 and to a detector circuit for exposure control 47. The digital signal, after a predetermined signal processing in the signal processing circuit 46, is stored in a memory 48 as a video signal, and it is taken out from an output terminal 49.
On the other hand, the digital signal supplied to the detector circuit for exposure control 47 is processed with field integration, intra-field peak detection, and so on, and then sent to a controller 50. The controller 50 includes, for example, a comparator, a loop filter, an AGC/iris control circuit, and so forth (not shown). An output signal from the controller 50 is converted to an analog signal in a D/A converter 51 and then supplied to the AGC circuit 44 as a control signal. The same signal is also supplied to the lens 41 through an iris driving circuit 52 as an open/close control signal for an iris provided in the lens 41. As a result, the gain of the iris of the lens 41 and the gain of the AGC circuit 44 are controlled to remain constant.
When the high-speed shutter mode of the electronic shutter of the video camera (for example, not less than 1/60 sec) is used, the video signal is supplied to the output terminal 49 for every field. Therefore, an iris gain and an AGC gain responsive to each field are obtained.
When, however, the low-speed shutter mode of the electronic shutter (for example, less than 1/60 sec; in this case, 1/15 sec) is used, the video signal of only one field (only the field labeled with 1) is supplied to the output terminal 49 for every four fields as shown in FIG. 2. In FIG. 2, the abscissa shows time and the ordinate shows amplitudes of the video signal. The vertical cycles in one field may be, for example, 60 Hz. In the period of three fields with no video signal supplied (the fields labeled with 2, 3 and 4), the iris gain and the gain of the AGC circuit 44 vary. That is, in order to prevent a decrease of the level of the output signal of the detector circuit for exposure control 47, the iris is opened to its maximum, and the AGC gain is maximized. Therefore, for example, when the video signal of the fifth field is input, it is necessary to decrease the iris gain and the AGC gain in accordance with the video signal. For this purpose, when a detected video signal of the fifth field is supplied to the controller 50, the iris gain and the AGC gain are decreased.
At the beginning of the video signal of the fifth field, the iris gain and the AGC gain are at maximum values which are transient to optimum iris and AGC gains for the fifth field. Therefore, it is impossible to obtain optimum iris and AGC gains to the video signal at this time.